Aircraft propellers comprise a plurality of propeller blades extending from a hub. During flight it is necessary, at times, to adjust the pitch of the blades relative to the air flow, to match flight conditions e.g. for take-off, climbing, descending, or landing in order to keep the propeller rotation speed constant.
In U.S. Pat. No. 5,199,850, pitch change is effectuated by a ballscrew and a ballscrew nut assembly that is responsive to a mechanical differential gearing that imports rotary motion to the ballscrew that, in turn, translates the ballscrew nut. The ballscrew nut is attached to the root of the propeller blade through a yoke, trunnion and a connecting link for increasing and decreasing the pitch of the blade. Each blade includes the trunnion and the connection link and the blades are moved in unison so that the blade angles are uniformly changed.
A similar system is taught in U.S. Pat. No. 7,296,969 and in U.S. Pat. No. 8,529,205.
In these conventional systems, all the blades are affected in the same way—i.e. the pitch change is the same for all blades.
A problem with conventional systems is propeller noise which can be high due to the blades having the same pitch and thus generating noise at the same frequency.
One attempt to reduce propeller noise is described in US 2015/0147181 in which the propeller blades are arranged in groups having different geometric characteristics. The different shapes will generate different noise signatures over a wider range of frequencies, providing a lower-noise propeller system. Such a system, however, is more manufacturing-intensive as different shaped blades are required in each propeller.
U.S. Pat. No. 8,109,728 teaches a system for adjusting the pitch angle of propeller blades where different groups of blades are rotated through different pitch angle ranges.